This invention relates to methods of and apparatus for the controlled cooling of a product. The invention is particularly concerned with the controlled cooling of specimens which are at least partially in liquid form. One particular application of the invention is to the freezing, e.g. for preservation, of biological materials.
It is well known to freeze biological and other materials, e.g. animal embryos, blood constituents etc., for the purpose of preservation in carrier media. The material is frozen in a liquid carrier medium at an accurately controlled rate, for example by the release of liquid nitrogen or some other coolant which is evaporated in the vicinity of the specimen. Suitable control equipment is used in the admittance of the coolant to maintain the appropriate cooling rate. When biological material is cooled, the critical rate, for instance typically -1.degree. C. per minute is commenced above the freezing point of the solution/suspension. One difficulty encountered in such controlled freezing procedures, for example in the freezing of embryos in liquid nitrogen, results from the sudden crystallisation of constituents of the material to be frozen, for example at temperatures between -7.degree. C. and -16.degree. C. Experience as shown that without special precautions crystallization during cooling takes place effectively simultaneously throughout the body of the specimen, with the resulting "shock" causing damage to the biological material. For this reason it is common practice in such cases to induce crystallization at the upper end of the ampoule or other container for the specimen, by physically contacting the ampoule or container with tongs or some other metal member which has previously been cooled in liquid nitrogen. The local crystallization which is thereby initiated then spreads progressively downwards through the ampoule or container and throughout the body of the specimen. Because this crystallization is more progressive, the survival rate of the biological material is substantially enhanced.
Another problem encountered in such controlled freezing procedures, and this is not limited to biological specimens, is that when crystallisation occurs the laten heat of fusion of the solution/suspension is released and the temperature of the liquid rises. There is also the potential problem of supercooling of the liquid, again with the danger of instant massive crystallization throughout the body of the liquid.
Among the disadvantages of the known methods described above, particularly the use of metal tongs, is the necessity of introducing mechanical movement within the cooling chamber, or in some cases momentary withdrawal of the specimen container, thus creating a risk of upsetting the control of the cooling rate. In addition, such manoeuvres are extremely inconvenient to the operator and require skill and expertise in order to achieve consistent satisfactory results.